This program will use the recently developed method of viscoelastometry (VE) to measure the size, conformation, and concentration (number) of mammalian chromosomal DNA molecules. By comparing changes in molecular size (Gamma), conformation (relative Tau and Gamma), and size concentration (ri) distribution, as functions of radiation dose and postirradiation treatment and/or repair, the rates of induction and repair of double-stand breaks (DSBs) will be determined. Cellular loss of reproductive integrity will be correlated with unrepaired DSBs per cell in order to determine if the unrepaired DSB is the major lethal lesion. Should such a correlation prove significant, the basis for a DNA based assay for cellular radiosensitivity will have been established. Since VE can detect the effects of clinically (and biologically) relevant doses (0.50 - 20 Gy), the above relationship will then provide a useful clinical assay. This assay will be tested in a competing renewal of this proposal. In addition, several biologically important points will be established by the data we propose to collect. The first of these is a determination as to whether mammalian chromosomal DNA consists of linear or circular molecules. At present nok data exist which directly demonstrate the DNA conformation. The data we propose to collect will demonstrate either the linear or the circular nature of relaxed chromosomal DNA in mammalian cells. This has far reaching consequences for our understanding of chromosome structure. The second point will be the determination of a general method for the determination of the D37, DSB and molecular conformation by the use of VE. The third point will be a test of the hypothesis that the shapes of the cell survival curves can be explained solely in terms of potentially lethal lesions (PLL) and their repair. We will examine the effects of postirradiation repair time followed by hypertonicity treatment to see if cell survival curves consistent with this interpretation are obtained. This has been done in one cell line but it is not clear if this is a general finding. The fourth point will be the determination of the number of unrepaired DSBs after each of the treatment used to assess PLL, and a test as to whether the PLL can be correlated with the DSB.